1. Field of the Invention
The present invention relates to an image forming apparatus. More particularly, the present invention relates to a printing media supply device for an image forming apparatus, which is capable of preventing multifeeding (that is, transferring several printing media into a main body of an image forming apparatus at one time) when the printing media in a printing media supply cassette is picked up.
2. Description of the Related Art
In general, an image forming apparatus includes a printing media supply device for supplying printing media, such as paper, fabric, film or the like, into a main body of the apparatus. Such a printing media supply device typically has a separate pick-up roller to pick up the printing media sheet by sheet.
One example of a printing media supply device of a conventional image forming apparatus is schematically shown in FIGS. 1 and 2. As shown in FIG. 1, a printing media supply device 100 includes a printing media supply cassette 110 having a space in which the printing media P is stacked, and a pick-up roller 120 which picks up the printing media P stacked in the printing media supply cassette 110 sheet by sheet to supply the printing media into the main body of the image forming apparatus.
As shown in FIG. 2, the printing media supply cassette 110 includes a base plate 111 detachably installed in a main body 1 of the image forming apparatus, and a stack of printing media P is stacked on the base plate 111. A first elastic member 112 pushes up one side of the base plate 111, and fingers 113 are installed at both side of the end of the base plate 111. The fingers 113 are interlocked with the base plate 111 so as to hinder movement of the printing media P and thus cause them to be supplied sheet by sheet.
The pick-up roller 120 is installed in an upper portion of the printing media supply cassette 110 to contact the printing media P stacked in the printing media supply cassette 110. The pick-up roller 120 has a pick-up member 121 for picking up the printing media P and a rotation member 122 coupled with a rotation shaft 123 while supporting the pick-up member 121.
The pick-up member 121 is provided on the rotation member 122, which is brought into contact with the printing media P, and exerts pressure on the printing media P. The pick-up member 121 is formed of, for example, a rubber material with a high elasticity to increase the frictional force on the printing media P. Further, on an outer circumferential surface of the pick-up member 121, saw-teethed protrusions 121a (see FIGS. 3A and 3B) are formed to increase the frictional force on the printing media P. Therefore, the pick-up member 121 picks up the printing media P by the frictional force generated when it is brought into contact with the printing media P and supplies them into the main body 1.
FIGS. 3A and 3B are views illustrating a process of feeding printing media by the pick-up roller. When a signal to pick up the printing media P is received from the image forming apparatus, a leading edge portion 122b of the pick-up roller 120 is first brought into contact with the printing media P, as shown in FIG. 3A, so that the printing media P is pressed in a direction indicated by the arrow A. Then, as shown in FIG. 3B, when the pick-up roller 120 is rotated in a direction indicated by the arrow B, the pick-up member 121 is brought into contact with the printing media P. The pick-up member 121 contacts the printing media P and transfers the uppermost printing media P in a direction indicated by the arrow C by frictional force, and supplies it into the main body 1 of the image forming apparatus.
In this process of feeding printing media, however, the pressure exerted on the top-most printing medium by the pick-up roller varies depending on the amount of printing media stacked in the printing media supply cassette. FIG. 4 is a view illustrating the operation of the printing media supply device shown in FIG. 1, depending on the amount of printing media. Referring to the drawing, the slope of the upper surface of the printing media P is closer to horizontal when the amount of printing media P is large (shown in solid lines) than when the amount of printing media is small (shown in dotted lines). Therefore, the pick-up roller 120 is brought into contact with the printing media P faster in the former case than in the latter case, so that the compression of the first elastic member 112 increases and the pressure exerted on the printing media P by the pick-up roller 120 increases. Accordingly, although the fingers 113 (that is, devices for separating the printing media) are installed in the conventional printing media supply cassette 110, there is a possibility of multifeeding because of the increased pressure exerted on the printing media P by the pick-up roller 120.
Accordingly, there is a need for an improved printing media supply device which is capable of preventing multifeeding regardless of the amount of printing media stacked in a printing media supply cassette.